Story highlights

1996: President Clinton signs the anti-gay marriage act, but later flips

2012: Same-sex marriage approved by statewide votes for the first time

Republican Sen. Rob Portman's flip-flop approval for same-sex marriage, is just the latest change of heart on the issue by conservatives.

Even Democrats like President Obama -- have turned around after opposing it. This change in attitude is just one of many milestones for the movement.

Here are five of the most important turning points in the same-sex marriage debate:

1993: In a landmark case, Hawaii's Supreme Court ruled that the state can't deny same-sex couples the right to marry unless it finds "a compelling reason" to do so. It orders the issue back to the state legislature, which then voted to ban gay marriage. This was one of earliest debates on the issue at the state level, and was a precursor to the legal battles nationwide. Today, domestic partnerships and civil unions for same-sex couples are legal in Hawaii.

1996: President Bill Clinton signed the Defense of Marriage Act, or DOMA, -- which defines marriage as a legal union between a man and a woman. The law denies federal benefits to same-sex couples in the nine states where gay marriage is legal. Clinton said he signed it because it would have tamped down calls for a constitutional amendment to ban gay marriage. Only 81 out of 535 members of Congress opposed DOMA. Fast-forward seventeen years to March 2013, when Clinton urged the Supreme Court to overturn DOMA. He explained: "As the president who signed the act into law, I have come to believe that DOMA is contrary to those principles and, in fact, incompatible with our Constitution."

Sen. Rob Portman backs same-sex marriage

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2004: President Bush championed a constitutional amendment that would outlaw gay marriage. It was needed, he said, to stop "activist judges" from redefining marriage. The idea found support among Senate conservatives, but its supporters couldn't gather enough votes. By the way, all this unfolded during a contentious presidential campaign. Democratic White House hopefuls Sens. John Kerry and John Edwards opposed the amendment, but they also were against creating a specific law making same-sex marriage legal.

2012: For the first time, voters approved same-sex marriage statewide at the ballot box. Similar measures had been rejected for years. Same-sex couples became free to marry in Maryland, Maine and Washington. Gay rights supporters also scored a smaller victory in Minnesota, where voters rejected a constitutional amendment to ban gay marriage. Interestingly, support for same-sex marriage came from a mixed coalition of voters. Before 2012, six states had already legalized gay marriage -- but via courts and legislatures -- not voters.

2013: For the first time, the Obama administration joined the legal battle against California's 2008 same-sex marriage ban. The Justice Department made it official in February when it filed a brief to the Supreme Court. The Obama administration urged the high court to invalidate the ban. Obama said that if he sat on the Supreme Court, he would vote to strike down Proposition 8. The court document expressed the president's evolution on the issue. In a short time he evolved from a backer of civil unions to a supporter of equality in marriage. Dozens of high-profile Republicans also argued in favor of same-sex marriage, in a court brief.